1. Field of the Invention
The present invention relates to a method and device for measuring the inside diameter of a part, and more particularly to a method and device for measuring the inside diameter of a metallic part using a capacitive probe.
2. Description of Related Art
Inside diameter measuring systems have been known and used in the prior art. The more common systems include a probe that makes physical contact between the probe and the inside surface of the part that is being measured. These systems are known as contact gaging systems.
Contact gaging has a number of drawbacks, including wear on the contact surfaces of the probe, deformation or damage to the part being measured, sensitivity to contact force, and complex setup procedures.
One example of a contact gaging system is disclosed in U.S. Pat. No. 4,417,816, issued on Nov. 29, 1983 to Kindl et al. The disclosed device uses a gage head that contacts the part being measured, while a laser interferometer tracks movement of the gage head.
Other devices that include noncontact systems are known for measuring inside surfaces of hollow metal parts. For example, U.S. Pat. No. 4,352,060, issued on Sept. 28, 1982 to Garpendahl et al., discloses a measuring means that is mounted for axial movement within a hollow, elongated part. An electric circuit creates a capacitance between the measuring means and the part being measured and senses any variations in this capacitance as the measuring means is moved axially through the part. Variations of the inside diameter of the part can be determined by the sensed variations in the capacitance reading.
U.S. Pat. No. 4,295,092, issued on Oct. 13, 1981 to Okamura, discloses an apparatus for detecting corrosion damage in pipes comprising a cylinder upon which two disks are mounted in parallel. The disks carry electric charges that are opposite in polarity. As the cylinder is moved through a pipe, locations where the pipe has been corroded cause a capacitance reading between the disks to vary.
U.S. Pat. No. 3,867,691, issued on Feb. 18, 1975 to Plessis, discloses a capacitive probe for measuring the roundness of the inside surface of a cylinder. The probe is supported by air pressure and rotated within the cylinder.
However, none of the noncontact measuring devices discussed above appear to be intended for measuring the inside diameter of a cylindrical part. The devices disclosed above appear to be intended for measuring variations or abnormalities in a cylindrical surface, as opposed to the actual diameter of the cylinder.